System and method for controlling fluid flow and temperature within a pumped two-phase cooling distribution unit

ABSTRACT

A control system for a cooling system utilizing a two-phase refrigerant includes a cooling distribution unit (CDU) having a CDU refrigerant supply line for supplying the refrigerant to at least one evaporator, and a CDU two-phase return line for returning the refrigerant to the CDU. A controller is operatively coupled to the CDU, the controller including a first control loop configured to maintain a constant pressure differential across the CDU refrigerant supply line and the CDU two-phase return line.

FIELD OF INVENTION

The present invention relates generally to heating and cooling controlsystems, and more particularly, to a system and method for controllingfluid flow and fluid temperature within a pumped two-phase coolingdistribution unit.

BRIEF DESCRIPTION OF THE DRAWINGS

One advanced method for cooling heat-generating devices is to use apumped two-phase cooling system. With reference to FIG. 1, an exemplarypumped two-phase cooling system 10 is illustrated. The system 10includes a liquid pump 12, one or more heat generating devices (notshown) cooled by one or more evaporators 14, a condenser 16 and anaccumulator tank 18 all in fluid communication via conduits 19 a-19 d.

In operation, the liquid refrigerant is partially boiled through the oneor more evaporators 14 producing a two-phase mixture that is transportedto the condenser 16 where it condenses back to a liquid. The evaporator14 can be in the form of a coiled tube surrounding the heat generatingdevice, a heat sink touching the heat generating device, a liquid torefrigerant heat exchanger removing heat from the heat generating devicevia an intermediary fluid, an air to refrigerant evaporator coolingheated air from the heat generating device, or another form of removingheat from the heat-generating device.

In many industries, including data centers, the portion of the two-phasecooling system that pumps and condenses the fluid is typically locatedremotely from the evaporator portion. The pumping and condensing portionis often times referred to as the cooling distribution unit (CDU). TheCDU also typically contains the control system, which manages the flowand temperature of the fluid.

SUMMARY OF THE INVENTION

There are several problems with conventional controllers currentlyavailable in CDU's. First, conventional controllers do not have anautomatic way to account for varying demand for fluid flow as parallelevaporating loops are added or subtracted. One example is the additionor removal of servers in an IT rack which each contain a fluidevaporating loop, where each fluid loop to each server is plumbed inparallel. In this example, as a server is added more fluid flow isneeded from the CDU and as a server is removed less fluid flow is neededfrom the CDU.

Secondly, conventional controllers do not have a way to automaticallyadjust the pump speed if cavitation exists. With pumped two-phasesystems, the pumps operate with the pump inlet fluid temperature veryclose to the boiling point of the fluid. During certain conditions suchas low heat load, no heat load, or a sudden loss of heat load, the pumpsare prone to cavitation, which is damaging to the pumping components.

Thirdly, conventional controllers do not have a method for maintainingtemperature control that is both fast enough and stable enough to workwith highly transient heat loads. The conventional approach is to use aProportional-Integral-Derivative (PID) control method to measure thefluid temperature (usually the two-phase return temperature to the CDU)and adjust a water valve or fan to hold that temperature nearlyconstant. The problem is that if the PID constants are set too high, thesystem becomes unstable under fast changes in heat load and if they areset too low, the system cannot react fast enough to the same transientconditions.

Lastly, there is a risk when using chilled water to condense thetwo-phase refrigerant that it may bring the refrigerant temperaturelower than the dew point temperature in the room. If this happens,condensation can form on the refrigerant plumbing and drip on to theelectrical devices, causing damage to the devices. The conventionalapproach is to monitor for condensation and alert the user if it occurs.

A system, apparatus and method in accordance with the present disclosureutilize one or more control methodologies to improve performance of apumped two-phase cooling system. The control methodologies include atleast one of flow control based on a pressure differential across theCDU, automatic cavitation control, refrigerant temperature control anddew point control.

According to one aspect of the invention, a control system for a coolingsystem utilizing a two-phase refrigerant includes: a coolingdistribution unit (CDU) including a CDU refrigerant supply line forsupplying the refrigerant to at least one evaporator, and a CDUtwo-phase return line for returning the refrigerant to the CDU; and acontroller (22) operatively coupled to the CDU, the controllercomprising a first control loop configured to maintain a constantpressure differential across the CDU refrigerant supply line and the CDUtwo-phase return line.

In one embodiment, the CDU includes: an accumulator tank; a refrigerantpumping portion having at least one pump; and a bypass valve, theaccumulator tank, refrigerant pumping portion and bypass valve in fluidcommunication with each other, wherein the bypass valve is operative todivert refrigerant flow from the at least one pump away from the atleast one evaporator and into the accumulator tank, the controllerconfigured to adjust a pump speed of the at least one pump and aposition of the bypass valve to maintain the CDU pressure differential.

In one embodiment, when refrigerant demand is below a predeterminethreshold, the controller is configured to command the at least one pumpto operate at a predetermined minimum speed, and to regulate the CDUpressure differential via the bypass valve.

In one embodiment, when the bypass valve is commanded to bypass aminimum amount of refrigerant the controller is configured to commandthe at least one pump to vary a speed to regulate the CDU pressuredifferential.

In one embodiment, the at least one pump comprises a plurality of pumps,and the controller is configured to place one of the plurality of pumpsin a standby mode and use another of the plurality of pumps to regulatethe CDU pressure differential.

In one embodiment, the at least one pump comprises a plurality of pumps,and the controller is configured to command each of the plurality ofpumps to operate at a reduced speed to regulate the CDU pressuredifferential.

In one embodiment, the controller is configured to use an output from asingle PID controller to command the plurality of fluid pumps.

In one embodiment, the controller further comprises a second controlloop configured to detect cavitation at the at least one pump, andautomatically vary a speed of the at least one pump upon detection ofcavitation.

In one embodiment, the second control loop is configured to detectcavitation by: converting refrigerant pressure at an inlet of the atleast one pump to a saturation temperature; comparing the saturationtemperature to an actual refrigerant temperature at the inlet of the atleast one pump to acquire an amount of subcool in the refrigerant;determining a minimum amount of subcool in the refrigerant that providescavitation-free operation for a given pump speed; and determiningcavitation is present based on a comparison of the amount of subcool inthe refrigerant and the minimum amount of subcool.

In one embodiment, the control system includes: a condenser(16) in fluidcommunication with the two-phase return line; a fluid valve (40) influid communication with the condenser and operative to provide acoolant to the condenser for cooling the refrigerant, wherein thecontroller includes a third control loop configured to regulate atemperature of the refrigerant by varying a coolant flow through thecondenser.

In one embodiment, the system includes: a condenser in fluidcommunication with the two-phase return line; a cooling device operativeto provide a cooling medium to the condenser for cooling therefrigerant, wherein the controller includes a third control loopconfigured to regulate a temperature of the refrigerant by varying aflow of the cooling medium through the condenser.

In one embodiment, the third control loop is configured to: generate arefrigerant temperature reference based on a refrigerant temperaturesetpoint and a tolerance value; convert the refrigerant temperaturereference to a corresponding pressure reference; and adjust coolant flowthrough the fluid valve based on a comparison of the correspondingpressure reference with an actual pressure at the CDU two-phase returnline.

In one embodiment, the controller comprises a fourth control loopconfigured to vary the refrigerant temperature setpoint based on acomparison of an ambient dew point and an actual refrigeranttemperature.

In one embodiment, the fourth control loop is configured to: calculatethe ambient dew point based on ambient temperature and ambient humidity;calculate a dew point reference based on a predetermined threshold valueadded to the calculated ambient dew point; and activate a dew pointfault when the fluid temperature is below the dew point reference.

In one embodiment, when a dew point fault is active, the fourth controlloop is configured to increase the refrigerant temperature setpoint by apredetermined value, and recalculate the ambient dew point.

In one embodiment, when a dew point fault is active and the refrigeranttemperature is above the dew point reference, the fourth control loop isconfigured to decrease the refrigerant temperature setpoint by apredetermined value, and recalculate the ambient dew point.

According to another aspect of the invention, a method for controlling acooling system utilizing a two-phase refrigerant, the cooling systemincluding a cooling distribution unit (CDU) having a CDU refrigerantsupply line for supplying the refrigerant to at least one evaporator,and a CDU two-phase return line for returning the refrigerant to theCDU, the method comprising maintaining a constant pressure differentialacross the CDU refrigerant supply line and the CDU two-phase returnline.

In one embodiment, the CDU includes a refrigerant pumping portion havingat least one pump, and a bypass valve is operative to divert refrigerantflow from the at least one pump away from the at least one evaporator,the method comprising adjusting a speed of the at least one pump and aposition of the bypass valve to maintain the CDU pressure differential.

In one embodiment, when refrigerant demand is below a predeterminethreshold, the method includes operating the at least one pump at apredetermined minimum speed using the bypass valve to regulate the CDUpressure differential.

In one embodiment, the method includes: detecting the occurrence ofcavitation at the at least one pump; and automatically varying a speedof the at least one pump upon detection of cavitation, whereinautomatically varying the speed includes converting refrigerant pressureat an inlet of the at least one pump to a saturation temperature;comparing the saturation temperature to an actual refrigeranttemperature at the inlet of the at least one pump to acquire an amountof subcool in the refrigerant; determining a minimum amount of subcoolin the refrigerant that provides cavitation-free operation for a givenpump speed; and determining cavitation is present based on a comparisonof the amount of subcool in the refrigerant and the minimum amount ofsubcool.

According to another aspect of the invention, a controller forcontrolling a two-phase cooling system includes: a processor and memory;and logic stored in the memory and executable by the processor, thelogic when executed by the processor configured to cause the processorto carry out the method described herein.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described andparticularly pointed out in the claims. The following description andthe annexed drawings set forth in detail certain illustrativeembodiments of the invention. These embodiments are indicative, however,of but a few of the various ways in which the principles of theinvention may be employed. Other objects, advantages and novel featuresof the invention will become apparent from the following detaileddescription of the invention when considered in conjunction with thedrawings

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the invention in accordance with the present disclosurecan be better understood with reference to the following drawings. Thecomponents in the drawings are not necessarily to scale, emphasisinstead being placed upon clearly illustrating the principles inaccordance with the present disclosure. Likewise, elements and featuresdepicted in one drawing may be combined with elements and featuresdepicted in additional drawings. Additionally, in the drawings, likereference numerals designate corresponding parts throughout the severalviews.

FIG. 1 is a schematic diagram illustrating a conventional two-phasecooling system.

FIG. 2 is a schematic diagram illustrating a two-phase cooling system inaccordance with the present disclosure.

FIG. 3 is a block diagram illustrating exemplary steps for carrying outa first control methodology in accordance with the present disclosure.

FIG. 4 is graph showing pump speed/valve position vs. PID output for asystem in accordance with the present disclosure utilizing one pump.

FIG. 5 is graph showing pump speed/valve position vs. PID output for asystem in accordance with the present disclosure utilizing two pumps.

FIG. 6 is graph showing required PID output vs. the number of installedparallel cooling loops.

FIG. 7 is a flow chart illustrating a pump cavitation controlmethodology in accordance with the present invention.

FIG. 8 is a flow chart illustrating a refrigerant temperature controlmethodology in accordance with the present invention.

FIG. 9 is graph showing water valve position vs. pressure setpoint inaccordance with the present disclosure.

FIGS. 10A and 10B illustrate steps for carrying out a dew point controlmethodology in accordance with the present invention.

DETAILED DESCRIPTION

A system, apparatus and method in accordance with the present disclosurefor controlling a cooling system, such as a two-phase cooling system,solve one or more of the issues listed above. In one embodiment, thesystem includes at least one and preferably four control loops that areseamlessly integrated together into an integrated control solution.

Referring to FIG. 2, an exemplary system 20 for controlling a two-phasecooling system is illustrated. Like the system 10 in FIG. 1, the system20 includes a liquid pump 12, one or more heat generating devices (notshown) cooled by one or more evaporators 14, a condenser 16 and anaccumulator tank 18, each in fluid communication via conduits 19 a-19 d.

A controller 22 executes one or more control algorithms in accordancewith the present disclosure to optimize performance of the coolingsystem. In one embodiment the controller 22 includes a processor andmemory communicatively coupled via a system bus, and logic stored inmemory and executable by the processor to cause the processor to carryout the method described herein. In another embodiment, the controller22 comprises an application—specific integrated circuit (ASIC) includinglogic configured to carry out the method described herein.

A pump inlet refrigerant temperature sensor 24 measures a temperature ofthe refrigerant entering the pump 12, an inlet pressure sensor 26 ameasures a pressure of the refrigerant at the inlet of the pump 12, andan outlet pressure sensor 26 b measures a pressure at the outlet of thepump 12 (i.e., the pressure exiting the CDU). A return line pressuresensor 27 measures a pressure in the refrigerant in the return line tothe CDU. A speed sensor 28 is operatively coupled to the pump 12 toprovide data corresponding to the pump speed. Such sensor may be ananalog sensor or digital sensor (e.g., tachometer, resolver, encoder,etc.), and may be directly coupled to the pump 12 or indirectly coupledto the pump 12 (e.g., to a motor (not shown) driving the pump). Althoughnot shown, the controller 22 is operatively coupled to a motor thatdrives the pump 12, and can vary a speed of the motor thereby varying aspeed of the pump 12. A bypass valve 29, which is controllable by thecontroller 22, couples an output of the pump back to the accumulatortank 18 via conduits 19 e and 19 f.

An ambient temperature sensor 30 and ambient humidity sensor 32 measurethe temperature and humidity of the ambient environment, respectively,and communicate the measurements to the controller 22. The ambientenvironment is the space immediately surrounding the CDU. If theevaporator 14 is located in a space different from the CDU, the ambientenvironment includes both spaces. One or more setpoint setting devices34 provide one or more setpoints to the controller 22 for regulating oneor more parameters of the system (e.g., refrigerant temperature, dewpoint safety margin temperature, chilled water temperature, CDU pressuredifferential, etc.). The temperature setting devices may be any analogor digital device capable of generating signal indicative of a desiredtemperature, or may be preset values configured in the controller 22(e.g., stored in memory of the controller).

Cooling water is provided from a cooling water source (not shown) to thecondenser 16 via a supply and return lines 36 and 38 to regulate atemperature of the refrigerant. A cooling water control valve 40 isoperatively coupled to the controller 22 and is operative to control theflow of cooling water through the condenser 16 based on a command fromthe controller 22.

As will be described in more detail below, the controller 22 executesone or more control methodologies in accordance with the presentdisclosure to control the two-phase cooling system. More particularly,the controller 22 may execute a first control methodology forautomatically controlling flow through the system based on a pressuredifferential between a liquid supply line and a two-phase return line. Asecond control methodology can prevent pump cavitation, while a thirdcontrol methodology controls a temperature of the refrigerant byadjusting water flow rate via a water control valve. Finally, a fourthcontrol methodology prevents the occurrence of condensation.

Pressure Differential Flow Control

A first control methodology that may be executed by the controller 22 isautomatic flow control via CDU pressure differential control. The flowcontrol loop works to maintain a constant pressure differential betweena CDU liquid supply connection 19 b and the CDU two-phase returnconnection 19 c. This is achieved by adjusting the speed of the pump 12(or pumps) as well as adjusting the position of the pump bypass valve29, which when open diverts flow from the pump 12 back to the tank 18within the CDU, thus bypassing the rest of the system.

The control loop is designed to control from zero flow up to full ratedflow of the system. Because the system may use positive displacementpumps, it is not feasible to block all flow while the pump 12 isrunning, as this would cause a large spike in pressure and could bedamaging to the system. Thus the bypass valve 29 may be needed duringoperating conditions when little or no flow is needed.

One example of why the system needs to operate with little or no flow iswhen most or all of the cooling loops are disconnected from the system.Such scenario may occur, for example, if removable servers withintegrated cooling loops are utilized. During initial installation orduring a period of maintenance, some or all of the servers may beremoved while the cooling system is still running. As each cooling loopis removed, less flow is needed from the CDU, therefore the flow outputof the CDU must be dynamic.

The control loop works off of a PID feedback loop with CDU dP being thefeedback signal. The dP setpoint is user selectable over a broad range(e.g., 20 psid to 100 psid). The control output is a 0-100% command thatis then converted to a pump speed command and a bypass valve positioncommand through a series of equations. FIG. 3 illustrates exemplary flowchart 50 in accordance with the present disclosure for implementing dPcontrol across the CDU. The steps illustrated in FIG. 3 may beimplemented by the controller 22.

Beginning at step 52, the pump outlet pressure sensor 26 b obtains apressure reading for the liquid supply line leaving the CDU 19 b andcommunicates the pressure reading to the controller 22, and at step 54the pressure sensor 27 obtains a pressure reading for the two-phasereturn line and communicates the pressure reading to the controller 22.Next at step 56 the controller 22 calculates a pressure differential dPbased on a difference between the measured supply line pressure andmeasured return line pressure. The calculated pressure difference isused by the controller as the differential feedback value dP. At step58, the controller 22 obtains a differential pressure setpoint value.Such setpoint value may be obtained, for example, via an operator inputdevice, communicated to the controller 22 via another device, calculatedbased on ambient and/or system conditions, or preset in the controller22.

Moving to step 60, the controller 22 calculates an error signal bycomputing the difference between the dP setpoint value as obtained instep 58 and the dP feedback value as obtained at step 56. The errorsignal then is provided to a control algorithm, such as a PID(proportional, integral and derivative) controller or the like, asindicated at step 62. The control algorithm generates an output thatattempts to cause the dP feedback value to approach the dP setpointvalue.

More particularly, and as indicated at step 64, the control algorithmcan calculate two different outputs. A first output corresponds to asetting for the bypass valve 29, e.g., between a minimum opening and amaximum opening of the bypass value. A second output corresponds to thespeed of the pump 12. An exemplary relationship between the commandvalue corresponding to pump speed and a command value corresponding tobypass valve position is shown in FIGS. 4 and 5.

As can be seen in FIGS. 4 and 5, at the lowest flow condition, zero flowleaving the CDU, the pump 12 (or pumps) must still run to maintain CDUdifferential pressure dP. However, due to mechanical and electricalconstraints with the pump 12, it may not be feasible to slow the pumpdown to near zero flow so it is commanded to run at an idle speed, whichis still within the pump's safe operating range. This sets the lowestvalue the pump 12 can run at while the system is running. At thiscondition, the bypass valve 29 must be at least partially open to allowa path for the fluid pumped by the pump 12 to flow. This value can be100% open or may be less depending on the design of the valve 29. In theexample above, there is little change in the effect of the valve 29between 60% and 100%, so the maximum valve position is set to 60%.

As more flow is needed, for example when more parallel loops areinstalled, the CDU pressure differential dP will drop since there aremore parallel loops requiring flow. As the actual dP drops, the controlscheme implemented by the controller at step 64 (e.g., a PID controlloop) will increase the output in an attempt to return the dP value backto the dP setpoint. As the output increases, the bypass valve 29position will begin to close, forcing less flow to bypass and more flowto leave the CDU. In the example shown above, this is achieved via alinear equation, however a non-linear equation could be used.

As more flow is required and the control algorithm continues to increasetowards 100%, at some point the valve 29 will come to its minimum value.The minimum value can be set at 0% open, or in the case of the exampleabove, it can be set at some value which is still partially open (e.g.,30% open). One reason for not fully closing the valve 29 is to alwayshave some bypass flow to help buffer large changes in flow required. Thepoint at which the bypass valve 29 has reached its minimum value can bereferred to as the switch point. At the switch point, as more flow isdemanded by the algorithm output, the pump 12 starts to speed up fromidle speed. In the case of a single pump 12, it will ramp from idlespeed to 100% of its rated flow as the algorithm output continues toclimb up to 100%. In the case of two pumps, each pump is set to rampfrom its idle speed to 50% speed at 100% algorithm output. The maximumspeed with two running pumps is reduced to half so that the total flowis approximately the same as with one running pump. In the case of morethan two pumps, a transfer equation can be implemented and tuned for thetotal number of pumps (e.g., there may be “n” transfer equations for “n”pumps), where the maximum speed of the pumps is limited to 100%/m, wherem is the number of pumps running at the time.

One advantage to having two pumps is system redundancy. If one pump(e.g., first) were to fail, the other (e.g., second) pump can turn onand/or change speed to maintain full cooling capability until the firstpump can be serviced. One method for controlling redundant pumps is tohave one pump on and the other in standby, where the standby pump isready to turn on and provide the same flow as the first pump should thefirst pump fail. The system can cycle between the two pumps within agiven time interval. Another method is to have both pumps running at areduced speed and quickly ramp one pump up to a higher speed should theother pump fail. This has several advantages, which include reducingwear on both pumps by running them slower and having a higher degree ofcertainty that the backup pump is functioning correctly prior to theprimary pump failing.

However, when two pumps are running at a reduced rate and one pumpsuddenly fails, the other pump needs to adjust very quickly to achievethe same CDU flow output and pressure differential. Since the controlalgorithm outputs a value to set the pump speed and bypass valve basedon CDU dP, it is ideal for the control algorithm to output a valueindeterminate of how many pumps are running. The “1 Pump” and “2 Pumps”curves as seen in FIGS. 4 and 5 have been tuned so that any change fromone pump to two pumps or two pumps to one pump along the algorithmoutput spectrum will result in negligible difference in CDU dP. This canbe seen in FIG. 6, which shows that as more cooling loops are installedthe PID output for a one-pump system and two-pump system areapproximately the same. The switch points can be tuned by utilizing acomputer simulation of the system or by testing. In the case of testing,the required PID output for a given set of pump speed and bypass valveconstraints can be found over a broad range of flow demand from zeroflow to max flow. A computer optimization routine can then be used tofind the optimal switch point values such that the difference in PIDoutput from 1 pump and 2 pump modes for the same flow demand isminimized.

Remaining at the same algorithm output regardless of whether one or twopumps are running is advantageous because the algorithm output does notneed to suddenly change when the system switches between two pumps andone or vice versa, only the transfer functions from algorithm output topump speed and bypass valve command change. This allows the controlsystem to essentially have advance knowledge of how to adjust to a pumpfailure without disrupting cooling operation to the electrical devicesbeing cooled. Minor differences in algorithm output can be adjusted viathe algorithm control loop with minimal impact on flow output.

One further improvement on this flow control method is to incorporate aramp rate in the dP setpoint any time it is changed, including when thesystem is first enabled. The rate, which can be hard-coded or userselectable, changes the setpoint at a fixed amount in a fixed time, forexample one psid per second.

This prevents large step changes in the setpoint, which can causeinstability in the control loop. This also allows the system togradually ramp up to the operating point from an initial value of 0 psidduring an initial enable event.

Automatic Cavitation Control

The second control methodology in accordance with the present disclosureis automatic cavitation control. Automatic cavitation control includesboth detecting when cavitation occurs and reacting to the cavitationevent in order to prevent damage to the pump 12 and return to an optimalpump speed as soon as possible.

In cavitation control mode the system overrides a pump speed valuecommanded via the pressure differential flow control loop (the firstcontrol loop) detailed above. Cavitation control mode will reduce thepump speed by a specified amount, for example 1%, then wait for aspecified amount of time, for example one second, and then repeat theloop by checking again for cavitation. To prevent running a pump tooslow, the pump speed may be limited to idle speed, even if cavitation isstill detected.

Once the system has sufficient subcool to be deemed free of cavitation,the control loop begins ramping the pump speed back up to its commandedspeed determined via the pressure differential flow control loop. If atany point during the process of ramping the pump speed back to thecorrect value cavitation is detected, the system will enter back intocavitation control mode and the control loop activate again. FIG. 7illustrates a flow chart 80 for implementing automatic cavitationcontrol in accordance with the present disclosure.

The automatic cavitation control algorithm 80 illustrated in FIG. 7detects when cavitation occurs by first acquiring the refrigerantpressure at the pump inlet, for example, via pressure transducer 26 alocated at the pump inlet as indicated at step 82. At step 84 thismeasured inlet pressure is converted to a corresponding saturationtemperature based on a known refrigerant saturation curve. At step 86the actual refrigerant temperature at the pump inlet is measured using,for example, pump inlet temperature sensor 24, and at step 88 a subcoolin the refrigerant is determined based on a comparison of therefrigerant saturation temperature obtained at step 84 and the actualrefrigerant inlet temperature obtained at step 86.

Pumps, based on their speed, need a different amount of subcool tooperate free of cavitation. At step 90 the required amount of subcoolfor the actual pump speed (which may be obtained from speed sensor 28)is determined. The required subcool may be determined, for example,using a transfer function that calculates a required subcool based onactual pump speed. The transfer equation can be a linear equation wherethe required subcool is determined based on pump speed and a minimumrequired subcool (slope and intercept) based on the design of the pump.A non-linear equation can also be used, as well as a more complextransfer function that uses additional parameters such as the pump inlettemperature.

At step 92 the required subcool value as determined in step 90 iscompared to the actual subcool value as determined at step 88. If theactual subcool value is not greater than the required subcool value amessage is output at step 94 to provide indication that the system is incavitation control mode. The method then moves to step 96 where theactual pump speed as determined from the speed sensor 28 (or a pumpspeed setpoint) is compared to a preset speed, e.g., idle speed. If theactual pump speed (or pump speed setpoint) is greater than the presetidle value then at step 98 a command is provided to a pump speedcontroller (not shown) to reduce the pump speed, e.g., reduce the speedby a predetermined percentage, step, etc. A purpose of step 98 is tobring the pump speed down to a level that requires a subcool less thanthe actual subcool at the pump inlet (thereby eliminating cavitation).The method then moves to step 106 where a time delay is introduced andthen the method moves back to step 82 and repeats. Moving back to step96, if the pump speed (or pump speed setpoint) is not greater than thepreset speed (minimum) then the pump speed cannot be further reduced andthe method moves to step 106 as described above.

Moving back to step 92, if the actual subcool value is greater than therequired subcool value, then cavitation is not present and the methodmoves to step 100 to determine if the pump speed had previously beenreduced due to cavitation control. If the pump speed had previously beenreduced, the method moves to step 102 where the pump speed is increasedby a predetermined percentage or step, and the method moves to step 106as discussed above. If at step 100 the pump speed had not previouslybeen reduced (or is at the desired setpoint speed), the method moves tostep 104 where cavitation mode is deactivated and the method moves tostep 106.

Although not shown, a hysteresis can be built into step 92 such that thesubcool must be above the threshold by a certain amount before thesystem can be deemed no longer cavitating.

It should be mentioned that other methods of cavitation detection canalso be used, such as determining the NPSHa (actual net positive suctionhead) instead of subcool, or detecting the onset of cavitation viapressure fluctuations, increase in sound or vibration on or around thepump, change in power required by the pump, etc. These alternativemethods would replace steps 82-92, however the rest of the logic wouldremain the same.

Refrigerant Temperature Control

The third control methodology in accordance with the present disclosureregulates the temperature of the refrigerant flowing through the coolingsystem. More particularly, the refrigerant temperature (i.e., thetwo-phase return fluid to the CDU) is regulated by varying a water flowrate through the condenser 16 via the water control valve 40.

Referring to FIG. 8, a flow chart 150 illustrating exemplary steps forcontrolling the refrigerant temperature is illustrated. The controlmethodology begins at step 152 by obtaining a desired temperaturesetpoint for the refrigerant. Such temperature setpoint may be obtained,for example, via operator entry, calculated by the controller 22 basedon ambient conditions, or stored (preset) in memory of the controller22. Next at step 154 a temperature setpoint tolerance is obtained viaoperator input or by retrieving a preset value (e.g., 3 degrees C.) frommemory of the controller 22. At step 156, a high temperature setpointand a low temperature setpoint are calculated based on the temperaturesetpoint obtained at step 152 and the temperature setpoint toleranceobtained at step 154. For example, the tolerance may be added to thetemperature setpoint to obtain the high temperature setpoint, and thetolerance may be subtracted from the temperature setpoint value toobtain the low temperature setpoint.

Next at step 158, the temperature setpoint, high temperature setpointand low temperature setpoint are converted to a corresponding pressurebased on a known refrigerant saturation curve for the refrigerant usedin the system. Pressure can be used interchangeably with temperaturebecause the returning refrigerant is a two-phase mixture of liquid andgas, or is a liquid that is nearly two-phase in the case of no heatrejection to the refrigerant to cause boiling.

At step 160 an equation (e.g., a linear equation) is derived from thelow pressure setpoint and the high pressure setpoint such that the watervalve 40 is at a low limit (e.g., 0% open) at the low saturationpressure and at a high limit (e.g., 100% open) at the high saturationpressure. FIG. 9 illustrates an exemplary linear equation for the watervalve position based on the low and high saturation pressure setpoints(which correspond to the low and high setpoints).

Next at step 162 the actual pressure at the two-phase CDU return isacquired using, for example, pressure sensor 27. Based on the actualreturn line pressure as obtained at step 162 and the equation derived atstep 160, the controller 22 determines a position for the water valve 40and commands the valve to the desired position as indicated at steps 164and 166. The valve 40 then moves to this position, and at step 168 adelay is introduced to allow the refrigerant temperature to change. Ahysteresis can be added to prevent continuous movement of the watervalve 40, which could lead to premature wear. At step 170 it isdetermined if a new temperature setpoint has been entered into thesystem (e.g., an operator changed the setpoint). If a new temperaturesetpoint has not been entered, the method moves to step 162 and repeats.However, if a new temperature setpoint has been entered, the methodmoves back to step 152 and repeats.

Although described above with respect to a water-cooled CDU utilizing aproportional water valve, the same logic can be applied to a CDU inwhich the heat is rejected by an air-cooled condenser with variablespeed fans. In this regard, the speed of the fans can be adjusted basedon the required amount of cooling instead of the position of the watervalve.

One advantage of the control method of FIG. 8 is that it is virtuallyinstantaneous, with the only lag time being the water valve speed.Because this method reacts very quickly to changes, there is little tono overshoot of the refrigerant temperature. Just as the system reactsquickly to sudden heat inputs, it also reacts very quickly to a suddenreduction in heat load. By quickly reducing water flow when the heatload is reduced, the system is better able to prevent over cooling ofthe refrigerant (which can lead to pump cavitation). The control loopuses pressure instead of temperature because the fluid pressure reactsmuch quicker to changes in the system compared to the fluid temperature,which has thermal mass and can be slow to react to changes.

One improvement on this control methodology, similarly to the automaticflow control method, is to add a ramp rate to the temperature setpointsuch that the controlling temperature cannot change too quickly. Therate, which can be preset or user selectable, changes the setpoint at afixed amount in a fixed time, for example 1 degree C. per minute. Thisensures that a change in the temperature setpoint does not cause asudden change in the water valve, which could lead to system cavitationand loss of performance.

Dew Point Control

The fourth control methodology in accordance with the present disclosureautomatically adjusts the refrigerant temperature setpoint mentionedabove if the system detects the dew point of the room has become tooclose to the current refrigerant temperature. The flow chart shown inFIGS. 10A and 10B provide exemplary steps 200 a and 200 b for carryingout dew point control in accordance with the present disclosure.

Beginning at step 202, the ambient air temperature for the area isacquired using, for example, ambient air temperature sensor 30, and atstep 204 the relative humidity of the area is obtained using ambienthumidity sensor 32. Next at step 206 the ambient dew point for the areais calculated based on the ambient temperature and the ambient humidityas obtained at steps 202 and 204. In calculating the dew point, amargin, for example 2 degrees C., may be added to the dew pointcalculated at step 208 to provide dew point threshold setpoint that hasa built-in a safety margin between the dew point of the room and thetemperature of the refrigerant. The margin may be a preset value storedin memory of the controller or it may be based on user entry.

Next at step 210 it is determined if a dew point fault condition isalready active. If a dew point fault condition is not active, the methodmoves to step 212 and obtains the refrigerant temperature at the pumpinlet, which may be obtained via temperature sensor 24. At step 214 thedew point temperature threshold as obtained in step 208 is compared tothe refrigerant temperature obtained at step 212. If the refrigeranttemperature at the pump inlet is not less than the dew point temperaturethreshold the method moves back to step 202. However, if the refrigeranttemperature at the pump inlet is less than the dew point temperaturethreshold, the method moves to step 216 and activates a dew point faultcondition, and at step 218 the current status of automatic dew pointcontrol is obtained. Dew point control status can be obtained, forexample, by reading an operator input (e.g. a selector switch or thelike) or provided by a supervisory control system.

At step 220 it is determined if dew point control is enabled or disabledbased on the dew point control status obtained at step 218. If automaticdew point control is not enabled, the method moves back to step 202.Disabling automatic dew point control may be desirable when the coolingsystem needs to maintain a set temperature and the user does not wantthe system to deviate from the temperature, even if dew point isdetected.

If automatic dew point control is enabled, the method moves to step 222where a refrigerant temperature setpoint is obtained (e.g., fromoperator input) and at step 224 a temporary refrigerant temperaturesetpoint is generated by adding a prescribed value, e.g., 0.1 degreesC., to refrigerant temperature setpoint using a prescribed ramp rate(e.g., 1 degree C. per minute). Next at step 226 a delay is introducedto allow the temporary refrigerant temperature setpoint to ramp to thenew value and/or to allow the system to react to the new setpoint, andthen the method moves back to step 202.

Moving back to step 210, if a dew point fault is active the method movesto step 228 and obtains the current status of automatic dew pointcontrol (e.g., as described above with respect to step 218). At step 230the controller 22 determines if automatic dew point control is enabledor disabled. If automatic dew point control is disabled, the methodmoves to step 232 and obtains the refrigerant temperature at the pumpinlet via temperature sensor 24, and at step 232 the refrigeranttemperature at the pump inlet is compared to the dew point temperaturethreshold as derived at step 208. If the refrigerant temperature at thepump inlet is less than the dew point temperature threshold the methodmoves back to step 202. However, if the refrigerant temperature at thepump inlet is not less than the dew point temperature threshold, themethod moves to step 236 and deactivates the dew point fault condition,and then moves back to step 202.

Moving back to step 230, if dew point control is enabled the methodmoves to step 238 and obtains the refrigerant temperature at the pumpinlet (e.g., via temperature sensor 24), and at step 240 the refrigeranttemperature at the pump inlet is compared to the dew point temperaturethreshold as derived at step 208. If the refrigerant temperature at thepump inlet is less than the dew point temperature threshold the methodmoves to step 242 where the temporary refrigerant temperature setpointis increased by a prescribed value, e.g., 0.1 degrees C., at aprescribed ramp rate (e.g., 1 degree C. per minute). Next at step 244 adelay is introduced to allow the temporary temperature setpoint to rampto the new value, and then the method moves back to step 202.

Moving back to step 240, if the refrigerant temperature at the pumpinlet is not less than the dew point temperature threshold, the methodmoves to step 246 where the refrigerant temperature setpoint isobtained, and at step 248 the refrigerant temperature setpoint iscompared to the temporary refrigerant temperature septoint. If thetemporary refrigerant temperature setpoint is not greater than therefrigerant temperature setpoint, the method moves to step 236 where thedew point fault condition is deactivated and then the method moves backto step 202. However, if the temporary refrigerant temperature setpointis greater than the refrigerant temperature setpoint, the method movesto step 250 where the refrigerant temperature at the pump inlet, whichmay be determined from temperature sensor 24, is compared to the dewpoint temperature threshold. If the refrigerant temperature at the pumpinlet is equal to the dew point temperature threshold, the method movesback to step 202. However, if the refrigerant temperature at the pumpinlet is not equal to the dew point temperature threshold, then themethod moves to step 252 where the temporary refrigerant temperaturesetpoint is decreased by a prescribed value (e.g., 0.1 degrees C.) at aprescribed ramp rate (e.g., 1 degree C. per minute). The method thenmoves to step 254 where a delay is introduced to allow the temporarytemperature setpoint to ramp to the desired value and/or the system toadjust to the new setpoint, and then the method moves back to step 202.

By using one or more of the four control methodologies described above,the CDU control system provides a highly stable, customizable controlsystem that is very robust to changing flow requirements, heat loads andexternal factors such as changing ambient conditions.

The scope of the present invention is not limited to only the controlmethodologies described above, but also includes other variations of theabove mentioned control methods that have not been specifically statedabove, but would be obvious to someone knowledgeable in the art.

Although the invention has been shown and described with respect to acertain embodiment or embodiments, it is obvious that equivalentalterations and modifications will occur to others skilled in the artupon the reading and understanding of this specification and the annexeddrawings. In particular regard to the various functions performed by theabove described elements (components, assemblies, devices, compositions,etc.), the terms (including a reference to a “means”) used to describesuch elements are intended to correspond, unless otherwise indicated, toany element which performs the specified function of the describedelement (i.e., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure which performs thefunction in the herein illustrated exemplary embodiment or embodimentsof the invention. In addition, while a particular feature of theinvention may have been described above with respect to only one or moreof several illustrated embodiments, such feature may be combined withone or more other features of the other embodiments, as may be desiredand advantageous for any given or particular application.

1. A control system for a cooling system utilizing a two-phaserefrigerant, comprising: a cooling distribution unit (CDU) including aCDU refrigerant supply line (19 b) for supplying the refrigerant to atleast one evaporator, CDU two-phase return line for returning therefrigerant to the CDU, a refrigerant pumping portion having at leasttwo pumps, and a bypass valve, wherein the refrigerant pumping portionand bypass valve are in fluid communication with each other, and thebypass valve is operative to divert refrigerant flow from the at leasttwo pumps away from the at least one evaporator; and a controlleroperatively coupled to the CDU, the controller comprising a firstcontrol loop configured to maintain a constant pressure differentialacross the CDU refrigerant supply line and the CDU two-phase return lineby adjusting a pump speed of the at least two pumps and a position ofthe bypass valve, said pump speed of each of the at least two pumpsbeing controlled via a single PID output, wherein a transfer equationcorresponding to a number of the at least two pumps is applied to thesingle PID output to limit a speed of each of the at least two pumps. 2.The control system according to claim 1, wherein the CDU comprises: anaccumulator tank in fluid communication with the refrigerant pumpingportion and bypass valve, wherein the bypass valve is operative todivert refrigerant flow from the at least one two pumps away from the atleast one evaporator and into the accumulator tank,
 3. The controlsystem according to claim 1, wherein when refrigerant demand is below apredetermine threshold, the controller is configured to command the atleast two pumps to operate at a predetermined minimum speed, and toregulate the CDU pressure differential via the bypass valve.
 4. Thecontrol system according to claim 3, wherein when the bypass valve iscommanded to bypass a minimum amount of refrigerant the controller isconfigured to command the at least two pumps to vary a speed to regulatethe CDU pressure differential.
 5. The control system according to claim3, wherein the controller is configured to place one of the plurality ofpumps in a standby mode and use another of the plurality of pumps toregulate the CDU pressure differential.
 6. The control system accordingto claim 3, wherein the controller is configured to command each of theplurality of pumps to operate at a reduced speed to regulate the CDUpressure differential.
 7. The control system according to claim 6,wherein the controller is configured to use an output from a single PIDcontroller to command the plurality of fluid pumps.
 8. The controlsystem according to claim 2, wherein the controller further comprises asecond control loop configured to detect cavitation at the at least twopumps, and automatically vary a speed of the at least two pumps upondetection of cavitation.
 9. The control system according to claim 8,wherein the second control loop is configured to detect cavitation by:converting refrigerant pressure at an inlet of the at least two pumps toa saturation temperature; comparing the saturation temperature to anactual refrigerant temperature at the inlet of the at least two pumps toacquire an amount of subcool in the refrigerant; determining a minimumamount of subcool in the refrigerant that provides cavitation-freeoperation for a given pump speed; and determining cavitation is presentbased on a comparison of the amount of subcool in the refrigerant andthe minimum amount of subcool.
 10. The control system according to claim1, further comprising: a condenser in fluid communication with thetwo-phase return line; a fluid valve in fluid communication with thecondenser and operative to provide a coolant to the condenser forcooling the refrigerant, wherein the controller includes a third controlloop configured to regulate a temperature of the refrigerant by varyinga coolant flow through the condenser.
 11. The control system accordingto claim 1, further comprising: a condenser in fluid communication withthe two-phase return line; a cooling device operative to provide acooling medium to the condenser for cooling the refrigerant, wherein thecontroller includes a third control loop configured to regulate atemperature of the refrigerant by varying a flow of the cooling mediumthrough the condenser.
 12. The control system according to claim 10,wherein the third control loop is configured to: generate a refrigeranttemperature reference based on a refrigerant temperature setpoint and atolerance value; convert the refrigerant temperature reference to acorresponding pressure reference; and adjust coolant flow through thefluid valve based on a comparison of the corresponding pressurereference with an actual pressure at the CDU two-phase return line. 13.The control system according to claim 12, wherein the controllercomprises a fourth control loop configured to vary the refrigeranttemperature setpoint based on a comparison of an ambient dew point andan actual refrigerant temperature.
 14. The control system according toclaim 13, wherein the fourth control loop is configured to: calculatethe ambient dew point based on ambient temperature and ambient humidity;calculate a dew point reference based on a predetermined threshold valueadded to the calculated ambient dew point; and activate a dew pointfault when the fluid temperature is below the dew point reference. 15.The control system according to claim 14, wherein when a dew point faultis active, the fourth control loop is configured to increase therefrigerant temperature setpoint by a predetermined value, andrecalculate the ambient dew point.
 16. The control system according toclaim 15, wherein when a dew point fault is active and the refrigeranttemperature is above the dew point reference, the fourth control loop isconfigured to decrease the refrigerant temperature setpoint by apredetermined value, and recalculate the ambient dew point.
 17. A methodfor controlling a cooling system utilizing a two-phase refrigerant, thecooling system including a cooling distribution unit (CDU) having a CDUrefrigerant supply line for supplying the refrigerant to at least oneevaporator, a CDU two-phase return line for returning the refrigerant tothe CDU, a refrigerant pumping portion having at least two pumps, and abypass valve operative to divert refrigerant flow from the at least twopumps away from the at least one evaporator, the method comprisingmaintaining a constant pressure differential across the CDU refrigerantsupply line and the CDU two-phase return line by adjusting a speed ofthe at least two pumps and a position of the bypass valve, whereinadjusting a speed of the at least two pumps includes generating a singlePID output for controlling a speed of each of the at least two pumps andapplying a transfer equation corresponding to a number of the at leasttwo pumps to the PID output to limit a speed of each of the at least twopumps.
 18. (canceled)
 19. The method according to claim 17 and any otherclaim, further comprising when refrigerant demand is below apredetermine threshold, operating the at least two pumps at apredetermined minimum speed using the bypass valve to regulate the CDUpressure differential.
 20. The method according to claim 17 and anyother claim, further comprising: detecting the occurrence of cavitationat the at least two pumps; and automatically varying a speed of the atleast two pumps upon detection of cavitation, wherein automaticallyvarying the speed includes converting refrigerant pressure at an inletof the at least two pumps to a saturation temperature; comparing thesaturation temperature to an actual refrigerant temperature at the inletof the at least two pumps to acquire an amount of subcool in therefrigerant; determining a minimum amount of subcool in the refrigerantthat provides cavitation-free operation for a given pump speed; anddetermining cavitation is present based on a comparison of the amount ofsubcool in the refrigerant and the minimum amount of subcool.
 21. Acontroller for controlling a two-phase cooling system, comprising: aprocessor and memory; and logic stored in the memory and executable bythe processor, the logic when executed by the processor configured tocause the processor to carry out the method according to claim 17.